For around $500 you can hit Newegg and get a Core i7-6700K, a motherboard, and 16 GB of DDR4 DRAM. Re-use your existing case, power supply, video card, etc. and run everything in MAME at full speed that's currently feasible to do so (and probably everything that will become feasible in the future). Bonus: your system will almost certainly be cooler/quieter than what you're upgrading from. The power per watt on the 6700K is the truth.